In the past, as a light source device used in a liquid crystal display device used in a mobile phone, a laptop computer, a liquid crystal television, a video camera, or the like, and a display device such as the backlit keys of a mobile phone, a backlit keyboard for a computer, a display switch of an electrical appliance, there is a direct type where a plurality of linear light sources such as fluorescent lights, or a plurality of point light sources, such as light emitting diodes, are individually arranged in a housing, and an edge light type where a linear light source or a point light source is arranged at a side surface of a light guide with a plate shape.
The light source device with the edge light type typically has a transparent member such as an acrylic resin plate with a rectangular plate shape as a light guide, light from a light source which is disposed to face a side surface is incident on the light guide from a side edge surface (a light incidence face), and by providing an emission mechanism formed on the front surface (a light emitting face) or the rear surface of the light guide or a light emitting means such as where light diffusion particles are included in the light guide, the incident light is emitted from the light emitting face. In a light guide such as this, the light which is incident from the side edge surface may lead to a reduction in the amount of light due to emission from not just the light emitting face but also the rear surface. As a result, in the light source device, a reflective sheet is arranged on an opposite side to the light emitting face of the light guide, and the emitted light is reused by being reflected from the rear surface and returned into the light guiding member. In this manner, it is possible to obtain a light source device which has superior luminance by the light from the light source being used with high efficiency.
In addition, in the typical light source device with the edge light type, since the light which is emitted from the light emitting face is emitted in a direction where the light guiding direction is horizontal with regard to the light emitting face, there is deviation in the peak of the emitted light with regard to the direction which is actually used in an electrical appliance and the use efficiency of the light is poor. As a result, in a case of being used in a display device, the emission direction is adjusted towards the front surface by finely controlling the shape of the emitting mechanism or also using an optical sheet such as a prism sheet or a diffusion sheet.
In recent years, thinner types of liquid crystal display devices, which are used in devices which are often carried around, such as portable laptop computers or mobile phones, have been in demand. Along with the demand for thinner types of liquid crystal display devices, thinner types of light source devices are also in demand. The effect of the light guide on the thickness of the light source device is high, and it can be said that making thinner types of light guide is the key to making thinner types of liquid crystal display devices and light source devices. On the other hand, the light guide has a purpose where uniform surface light emission in the light source device is an aim as well as a purpose of supplying and emitting light to a particular position. In terms of both purposes, the prevention of the loss of light which is incident in the light guide and the light source device emitting light with high luminance are demanded in the light guide. For example, there is proposed a light guide (for example, Patent Literature 1) for a backlight of a key pad which is thin while being able to supply light to a particular position and emit light with high efficiency using a single layer of a polycarbonate sheet being the light guide and through being formed in a pattern with a convex shape by the front surface of the polycarbonate sheet being processed by hot stamping, injection molding, micro blasting, or the like, and a light guide (for example, Patent Literature 2) for a backlight for a keyboard which emits light by a dot pattern being provided on the front surface of a surface which faces the light emitting face and light which is guided in the light guide being reflected or scattered to the light emitting face side.
There is proposed a light guide (for example, Patent Literature 3) where light is emitted by a flat optical fiber which is provided with a core layer and a cladding layer with a refractive index lower than the core layer being the light guide, the light being efficiently confined in the light guide using the total reflection of the core-cladding interface, and forming a discontinuous portion in the cladding layer on the light emitting face in a position where light is to be emitted. The light guide which has a core-cladding configuration suppresses light leaking from the light guide, and use efficiency of the light which is incident from the light incidence face is achieved.